Charge pump circuits are typically utilized in voltage controlled oscillator (VCO) applications wherein the charge pump circuit is responsive to Up and Down input signals to provide a current at the output of the charge pump that is used to either increase or decrease a voltage across a capacitor that is typically coupled to the output of the charge pump wherein it is understood that the voltage across the capacitor controls the frequency of oscillation of the VCO.
Charge pump circuits typically include at least three current sources; a first current source that is always on and has its output coupled to source current to the output of the charge pump; and second and third switched current sources which both have their outputs coupled to sink current from the output of the charge pump.
In general, a charge pump can essentially operate in three different modes. The first mode is when the second and third current sources are disabled such that the first current source sources current to the output of the charge pump thereby increasing the voltage across an external capacitor that is typically coupled to the output of the charge pump. A second mode is when the second current source is enabled while the third current source is disabled such that the second current source cancels the current sourced from the first current source thereby providing a zero current source at the output of the charge pump. Finally, a third mode is when both the second and third switched current sources are enabled such that one of the current sources cancels the current sourced from the first current source and the other current source then sinks a predetermined current from the output of the charge pump thereby decreasing the voltage across the external capacitor.
One problem with the above described charge pump is that as the voltage at the output of the charge pump varies, the current sourced from the first current source also varies because its output is coupled to the output of the charge pump. However, the current sunk from the second and third current sources do not vary. As a result, this leads to error currents appearing at the output of the charge pump which can eventually cause timing and phase errors within a phase-locked loop.
One method to decrease this net current error is to increase the value of the resistor in the emitter leg of the PNP transistor of the first current source. This has the effect of making the change in the base-emitter voltage of the PNP transistor a smaller percentage of the total voltage across the resistor. However, this solution limits the overall operating range of the charge pump, and does not account for the alpha changes of the PNP transistor wherein as the collector-emitter voltage of the transistor changes, the alpha of the transistor also changes which subsequently causes a change in the collector current of the transistor.
Hence, there exists a need for an improved charge pump circuit that adjusts the lower pull down current sources to reflect the current changes of the pull up current source which are due to a changing voltage at the output voltage of the charge pump.